Better OLEDs, Perhaps, Thanks to a Holey Electrode

The makers of organic LEDs face a challenge. The devices are constructed from a series of organic semiconductor layers sandwiched between a cathode and anode, which must be conductive and transparent. Unfortunately, the best and least expensive conductors — metals — are opaque.

Now graduate student Myung-Gyu Kang and associate professor of electrical engineering and computer science L. Jay Guo, both at the University of Michigan in Ann Arbor, have used the technique called nanoimprinting to create an anode from a semitransparent metal mesh film. The process allows transparency and conductivity to be adjusted almost independently of each other.

On the top are scanning electron micrographs of nanoimprint molds that researchers constructed and used to form the semitransparent metal electrodes on the bottom. The resulting material may be used to replace indium tin oxide as the anodes of organic LEDs. Images courtesy of L. Jay Guo, University of Michigan.
As a result, the researchers were able to construct organic LEDs with nontraditional anodes. “We demonstrated working organic LED devices by using the metal mesh electrodes made of different metals — including copper, which is very inexpensive,” Guo explained.

The material of choice for anodes has traditionally been the transparent conductor indium tin oxide. However, indium has experienced a price jump recently. Moreover, both indium and oxygen can migrate into the organic semiconductors, hurting overall device performance over time. In addition, indium tin oxide is not the most efficient injector of holes, and its thin-film resistance is high — factors that limit devices. Finally, the higher refractive index of indium tin oxide as compared with that of the organic materials results in only 25 percent of generated photons ever making it out of the device.

After early attempts to pursue the unconventional approach of exploiting metal surface plasmon resonance, the investigators tried something simpler to enhance metal transparency: They opened up the metal film as much as possible without compromising electrical performance.

To accomplish that goal, they turned to nanoimprinting, using narrow linewidth molds to transfer sets of lines into a metal film. They developed their own mold-creation techniques, paying particular attention to certain requirements. “Fabricating the narrow linewidth mold with the ability of adjusting certain parameters was very important to the whole process,” Guo said.

With this method, they created a subwavelength metal mesh on glass. Using 200-nm lines and 500-nm spaces, the researchers achieved a 69 percent transmittance with a 40-nm-thick gold film. With 120-nm lines and 580-nm spaces, they hit 81 percent. The unpatterned metal film, in contrast, had a total of only 6.7 percent transmittance.

By starting with a film of a different thickness, the researchers created wires that were more conductive but had little effect on transmissivity, particularly if the lines were made narrower at the same time. For example, they boosted the conductivity threefold while cutting transmissivity only 5 percent.

Using the molds, the scientists also made mesh electrodes with rectangular grids, an arrangement that they felt would guard against defects in the fabrication process. They used these mesh electrodes to build organic LEDs that demonstrated performance comparable to that of traditional devices. With optimization, they concluded, the new electrodes could replace indium tin oxide as the anode.

According to Guo, ongoing research aims to further refine the electrode designs for organic LEDs and other applications. “We are also exploiting the new electrode structures in organic solar cells to replace indium tin oxide, and our initial results are very promising,” he said.